feat(rdd): Windows LockFileEx implementation — real byte-range locks

Replace the no-op Windows lock stub with actual kernel32 LockFileEx /
UnlockFileEx calls via syscall.LazyDLL (zero external dependency).

- LOCKFILE_EXCLUSIVE_LOCK | LOCKFILE_FAIL_IMMEDIATELY for non-blocking
  semantics matching Clipper FLOCK() → .F.
- Same lock region layout as POSIX: header region for FLOCK, record
  offsets for DBRLOCK — compatible across platforms
- Handles returned as syscall.Handle from os.File.Fd()

Note: full Windows cross-compile still blocked by unrelated issues
(mmap in cdx/ntx, termios in debugcli.go). The lock code itself
compiles cleanly with //go:build windows.

Also updates gap-analysis.md to reflect Windows lock status.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/gap-analysis.md

@@ -108,7 +108,11 @@ per-fd, so same-process tests would be meaningless).
 
 ### Limitations
 
-- **Windows**: still a no-op stub. `LockFileEx` wrapper needed (~1 day).
+- **Windows**: real `LockFileEx`/`UnlockFileEx` implementation in
+  `locks_windows.go` using `kernel32.dll` direct syscalls (no external
+  dependency). Note: full Windows cross-compile requires separate fixes
+  for mmap (`hbrdd/cdx`, `hbrdd/ntx`) and termios (`hbrt/debugcli.go`)
+  which are unrelated to locking.
 - **Advisory only**: processes that don't call `FLOCK`/`DBRLOCK` bypass
   protection. Same as Harbour and Clipper — this is expected behavior.
 - **No timeout**: Harbour's `HB_SET_LOCKRETRY` is not honored. Callers

hbrdd/dbf/locks_windows.go

@@ -3,45 +3,191 @@
 
 //go:build windows
 
-// Windows file locking stub for DBF shared mode.
-// TODO: implement with LockFileEx (LOCKFILE_EXCLUSIVE_LOCK | LOCKFILE_FAIL_IMMEDIATELY).
-// For now, behaves like the previous no-op (always succeeds).
+// Windows file locking for DBF shared mode.
+// Uses kernel32 LockFileEx / UnlockFileEx for byte-range exclusive locks,
+// matching Harbour's hb_fsLockLarge on Windows.
+//
+// Lock region layout (identical to the POSIX implementation):
+//   - FLOCK:  [0, HeaderLen+1)            file-wide exclusive
+//   - RLOCK:  [RecordOffset(n), RecordLen) per-record exclusive
+//
+// Non-blocking: LOCKFILE_FAIL_IMMEDIATELY returns immediately if another
+// process holds a conflicting lock — matching Clipper FLOCK() → .F.
 
 package dbf
 
-func (a *DBFArea) LockFile() (bool, error) {
-	a.fileLocked = true
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+const flockOffset = int64(0)
+
+func flockLen(a *DBFArea) int64 {
+	return int64(a.header.HeaderLen) + 1
+}
+
+var (
+	modkernel32      = syscall.NewLazyDLL("kernel32.dll")
+	procLockFileEx   = modkernel32.NewProc("LockFileEx")
+	procUnlockFileEx = modkernel32.NewProc("UnlockFileEx")
+)
+
+// Windows constants
+const (
+	_LOCKFILE_EXCLUSIVE_LOCK   = 0x00000002
+	_LOCKFILE_FAIL_IMMEDIATELY = 0x00000001
+	_ERROR_LOCK_VIOLATION      = 33
+)
+
+func splitOffset(off int64) (lo, hi uint32) {
+	return uint32(off & 0xFFFFFFFF), uint32(off >> 32)
+}
+
+// tryLock acquires an exclusive byte-range lock via LockFileEx.
+// Non-blocking: returns (false, nil) on contention.
+func tryLock(handle syscall.Handle, start, length int64) (bool, error) {
+	oLo, oHi := splitOffset(start)
+	lLo, lHi := splitOffset(length)
+	ol := &syscall.Overlapped{
+		Offset:     oLo,
+		OffsetHigh: oHi,
+	}
+	flags := uint32(_LOCKFILE_EXCLUSIVE_LOCK | _LOCKFILE_FAIL_IMMEDIATELY)
+	r1, _, err := procLockFileEx.Call(
+		uintptr(handle),
+		uintptr(flags),
+		0, // reserved
+		uintptr(lLo),
+		uintptr(lHi),
+		uintptr(unsafe.Pointer(ol)),
+	)
+	if r1 == 0 {
+		// Call failed
+		if errno, ok := err.(syscall.Errno); ok && errno == _ERROR_LOCK_VIOLATION {
+			return false, nil // another process holds it
+		}
+		return false, err
+	}
 	return true, nil
 }
 
+// unlockRange releases a previously acquired byte-range lock via UnlockFileEx.
+func unlockRange(handle syscall.Handle, start, length int64) error {
+	oLo, oHi := splitOffset(start)
+	lLo, lHi := splitOffset(length)
+	ol := &syscall.Overlapped{
+		Offset:     oLo,
+		OffsetHigh: oHi,
+	}
+	r1, _, err := procUnlockFileEx.Call(
+		uintptr(handle),
+		0, // reserved
+		uintptr(lLo),
+		uintptr(lHi),
+		uintptr(unsafe.Pointer(ol)),
+	)
+	if r1 == 0 {
+		return err
+	}
+	return nil
+}
+
+// winHandle returns the Windows HANDLE for the data file.
+func (a *DBFArea) winHandle() syscall.Handle {
+	return syscall.Handle(a.dataFile.Fd())
+}
+
+// LockFile tries to acquire an exclusive FLOCK on the DBF file.
+func (a *DBFArea) LockFile() (bool, error) {
+	if a.dataFile == nil {
+		return false, fmt.Errorf("file not open")
+	}
+	if a.fileLocked {
+		return true, nil
+	}
+	ok, err := tryLock(a.winHandle(), flockOffset, flockLen(a))
+	if err != nil {
+		return false, err
+	}
+	if ok {
+		a.fileLocked = true
+	}
+	return ok, nil
+}
+
+// UnlockFile releases the file-wide lock held by this workarea.
 func (a *DBFArea) UnlockFile() error {
+	if a.dataFile == nil || !a.fileLocked {
+		return nil
+	}
+	if err := unlockRange(a.winHandle(), flockOffset, flockLen(a)); err != nil {
+		return err
+	}
 	a.fileLocked = false
 	return nil
 }
 
+// LockRecord tries to acquire an exclusive lock on a single record.
 func (a *DBFArea) LockRecord(recNo uint32) (bool, error) {
-	if a.lockedRecs == nil {
-		a.lockedRecs = make(map[uint32]bool)
+	if a.dataFile == nil {
+		return false, fmt.Errorf("file not open")
 	}
 	if recNo == 0 {
 		recNo = a.recNo
 	}
-	a.lockedRecs[recNo] = true
-	return true, nil
+	if recNo == 0 {
+		return false, nil
+	}
+	if a.lockedRecs != nil && a.lockedRecs[recNo] {
+		return true, nil
+	}
+	start := a.header.RecordOffset(recNo)
+	length := int64(a.header.RecordLen)
+	ok, err := tryLock(a.winHandle(), start, length)
+	if err != nil {
+		return false, err
+	}
+	if ok {
+		if a.lockedRecs == nil {
+			a.lockedRecs = make(map[uint32]bool, 16)
+		}
+		a.lockedRecs[recNo] = true
+	}
+	return ok, nil
 }
 
+// UnlockRecord releases a single record lock. recNo == 0 unlocks ALL.
 func (a *DBFArea) UnlockRecord(recNo uint32) error {
-	if a.lockedRecs == nil {
+	if a.dataFile == nil {
 		return nil
 	}
 	if recNo == 0 {
+		if a.lockedRecs == nil {
+			return nil
+		}
+		length := int64(a.header.RecordLen)
+		for r := range a.lockedRecs {
+			start := a.header.RecordOffset(r)
+			_ = unlockRange(a.winHandle(), start, length)
+		}
 		a.lockedRecs = nil
 		return nil
 	}
+	if a.lockedRecs == nil || !a.lockedRecs[recNo] {
+		return nil
+	}
+	start := a.header.RecordOffset(recNo)
+	length := int64(a.header.RecordLen)
+	if err := unlockRange(a.winHandle(), start, length); err != nil {
+		return err
+	}
 	delete(a.lockedRecs, recNo)
 	return nil
 }
 
+// releaseAllLocks is called on Close to release any held locks.
 func (a *DBFArea) releaseAllLocks() {
 	_ = a.UnlockFile()
 	_ = a.UnlockRecord(0)